Selective hydrate production with co2 and controlled depressurization

ABSTRACT

The present invention relates to an improved method for recovering hydrocarbons trapped in hydrate formations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit under 35 U.S.C. Section 119(e) to U.S. Provisional Patent Ser. No. 61/406,261 filed on Oct. 25, 2010 the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an improved method for recovering hydrocarbons trapped in hydrate formations.

BACKGROUND OF THE INVENTION

A number of hydrocarbons, especially lower boiling-point light hydrocarbons, in formation fluids or natural gas, are known to form hydrates in conjunction with the water present under a variety of conditions—particularly at a combination of lower temperature and higher pressure. The hydrates are solid crystalline compounds which co-exist with the surrounding porous media or natural gas fluids. Any solids in formation or natural gas fluids are at the least a nuisance for production, handling, and transport of these fluids. It is not uncommon for solid hydrates to cause plugging and/or blockage of pipelines or transfer lines or other conduits, valves and/or safety devices and/or other equipment, resulting in shutdown, loss of production, and risk of explosion or unintended release of hydrocarbons into the environment either on-land or off-shore. Accordingly, hydrocarbon hydrates have been of substantial interest as well as concern to many industries, particularly the petroleum and natural gas industries.

Natural gas hydrates are in a class of compounds known as clathrates, and are also referred to as inclusion compounds. Clathrates consist of cage structures formed between a host molecule and a guest molecule. Gas hydrates are generally composed of crystals formed by water host molecules surrounding the hydrocarbon guest molecules. The smaller or lower-boiling hydrocarbon molecules, particularly C₁ (methane) to C₄ hydrocarbons and their mixtures, are often the most problematic in the oil and gas industry because they form in hydrate or clathrate crystals under a wide range of production conditions. Even certain non-hydrocarbons such as carbon dioxide and hydrogen sulfide are known to form hydrates under the proper conditions. Beyond being a problem for production of hydrocarbons, hydrates are being looked at as a possible energy source.

Known production strategies of hydrocarbons from hydrates have mainly focused on dissociation of the hydrate to release the trapped hydrocarbons. These known methods of dissociation of hydrates are based on shifting the thermodynamic equilibrium, which can be achieved by: increasing the system temperature above the temperature of hydrate formation at a specified pressure; decreasing the system pressure below the pressure of hydrate formation at a specified temperature; or injecting inhibitors such as methanol to shift the pressure-temperature equilibrium. However, there are two major problems with these known methods. First, these known methods can require a large amount of energy to be added to the system, especially in heating methods, resulting in a high cost of extraction. Second, they destabilize hydrate formations because both depressurization and heating cause the hydrate to dissociate. This can lead to the destabilization and/or collapse of sediments that contain hydrates and other nearby subterranean reservoirs. Because gas hydrates are often located near oil and natural gas deposits, such instability during extraction can result in problems with the extraction of oil and natural gas.

Of the known hydrate production methods being explored, hydrate depressurization provides significant economical and process benefits over thermal heating or inhibitor injection. Hydrate bearing layers are subjected to pressure from the hydrostatic pressure of the overburden. Hydrate depressurization decreases the pressure at the hydrate interface, so that the hydrate equilibrium temperature is below that of the surroundings temperature, inducing dissociation. However, the hydrate production from depressurization suffers from several limitations including, but not limited to: significant production of water, endothermic cooling, and loss of geomechanical stability as hydrates dissociate. Furthermore, depressurizing inside the well will not necessarily lead to depressurization of the entire methane hydrate-bearing layer.

An alternative process for the extraction of natural gas hydrates has been identified using a releasing agent. The releasing agent is preferably a hydrate-forming fluid which is thermodynamically more stable under system conditions than the natural gas hydrate. This has been most studied with carbon dioxide (often in liquid form) as the releasing agent. Injection of carbon dioxide has been shown to release the natural gas in the hydrate while simultaneously sequestering the carbon dioxide in the hydrate cages. This process only requires the injection of a releasing agent and is therefore less energy intensive then conventional methods. In addition, because the releasing agent enters the hydrate and exchanges with natural gas, bulk hydrate dissociation is not thought to occur, mitigating the destabilization risk posed by hydrate dissociation during production.

Therefore, a need exists for a method that mitigates the significant production of water, the endothermic cooling, and the loss of geomechanical stability as hydrates dissociate during the hydrate depressurization process while simultaneously providing environmentally and economically positive consequences of sequestering carbon dioxide in the form of gas hydrate.

SUMMARY OF THE INVENTION

In an embodiment, a method for the producing hydrocarbons from a subterranean formation containing gas hydrates, the method includes: (a) drilling a well into a subterranean formation; (b) introducing a releasing agent into the well in a controlled manner to partially depressurize the well, wherein the releasing agent is more thermodynamically stable than the gas hydrates present in the formation; (c) causing the releasing agent to contact the gas hydrates, thereby releasing the hydrocarbons in the well without melting the gas hydrate; (d) selectively substituting the releasing agent for the hydrocarbons to thereby release the hydrocarbons from solid-state hydrate structure s without melting the gas hydrates, thereby providing substituted hydrates comprising the releasing agent bound with the solid-state hydrate structures that form hydrates, thereby releasing the hydrocarbons into the well without melting the gas hydrates; and (e) reducing the pressure in the well to selectively dissociate the hydrates with a desired methane content.

In another embodiment, a method for the producing hydrocarbons from a subterranean formation containing gas hydrates the method comprising: (a) drilling a well into a subterranean formation; (b) introducing a releasing agent into the well in a controlled manner to partially depressurize the well, wherein the releasing agent is more thermodynamically stable than the gas hydrates present in the formation, wherein the releasing agent is selected from a group consisting of carbon dioxide, ethane, xenon, hydrogen sulfide, and mixtures thereof; (c)causing the releasing agent to contact the gas hydrates, thereby releasing the hydrocarbons in the well without melting the gas hydrate; (d) selectively substituting the releasing agent for the hydrocarbons to thereby release the hydrocarbons from solid-state hydrate structure s without melting the gas hydrates, thereby providing substituted hydrates comprising the releasing agent bound with the solid-state hydrate structures that form hydrates, thereby releasing the hydrocarbons into the well without melting the gas hydrates; and (e) reducing the pressure in the well to selectively dissociate the hydrates with a desired methane content.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram showing an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to embodiments of the present invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not as a limitation of the invention. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations that come within the scope of the appended claims and their equivalents.

Methane hydrate conversion to carbon dioxide hydrate requires an understanding of hydrodynamics of carbon dioxide injection and transport to the methane hydrate accumulation; along with thermodynamics of formation and dissociation of the hydrates of methane

In order to recover sequestered methane gas from within a subterranean formation containing a gas hydrates, one or more wells are drilled into the subterranean formation. A casing string is cemented within the subterranean formation and one or more windows or perforations are opened directly into the area of the formation containing the gas hydrates. At this point, the releasing agent (e.g., carbon dioxide) is injected into the formation. The releasing agent contacts the gas hydrate, resulting in the releasing agent spontaneously (i.e., without the need for added energy) replacing the gas within the formation without requiring a significant change in the temperature, pressure, or volume of the hydrate. As used herein, the releasing agent is a compound that forms a thermodynamically more stable than the gas hydrates originally contained within the well. In an embodiment, the releasing agent is selected from a group consisting of carbon dioxide, ethane, xenon, hydrogen sulfide, and mixtures thereof. In another embodiment, the releasing agent is gaseous carbon dioxide. In another embodiment, the releasing agent is liquid. In yet another embodiment, the releasing agent is liquid carbon dioxide.

As the hydrate becomes enriched in the releasing agent, the hydrate releasing agent mixture becomes more stable based on the thermodynamic pressure-temperature relationship (i.e. for a given temperature, the hydrate will remain stable at lower pressures). The stability of the hydrate refers to the pressure at which it dissociates at a given pressure. The more stable the hydrate, the lower this pressure will be.

After an initial period of releasing agent exchange, the pressure of the well can be reduced to selectively dissociate only hydrates with a desired methane content. The original in-place hydrate is less stable than the hydrate following exchange with the releasing agent. Further, hydrates will become increasingly stable based on the exposure conditions to the releasing agent (e.g. exposure time, composition of releasing agent in pore space). Therefore, based on the exposure time of the hydrate to the releasing agent, the pressure in which the well is depressurized to will only cause partial hydrate dissociation. With hydrate dissociation selectively limited, the amount of free water created (due to hydrate dissociation) is also simultaneously controlled by the well operating pressure. Thus, part of the hydrate is selectively dissociated with the simultaneous reformation of the releasing agent hydrate. This method minimizes loss of geomechanical stability which can occur in known depressurization production techniques.

Turning now to the drawings wherein like numerals indicate like parts, FIG. 1 discloses a pictorial representation of one operating context of the invention. FIG. 1 illustrates an oil or natural gas well 18 that has been modified to employ the present inventive methodology. Well 18 generally comprises a superstructure 20 and a casing string located in the well down to the reservoir interval of interest 22. Well 18 was previously used to produce oil and/or gas from a subterranean reservoir 24 via lower perforations 26 in casing 22. In an embodiment, the well is drilled for hydrate production. After production of the oil and/or gas from subterranean reservoir 24 has been completed, a plug 28 is inserted into casing 22 above perforations 26 and immediately below a gas hydrate formation 30. The casing 22 and the perforations 26 create a subterranean channel, which provides access to the hydrates in question. In an embodiment, the gas hydrate is a methane hydrate.

Upper perforations 32 are created in casing 22 above plug 28 and proximate to hydrate formation 30. The perforations 32 open up the well bore to the reservoir through the casing, which allows on to reduce the pressure in the hydrate formation 30 around the well 18 and induce an initial dissociation of the gas hydrates. As the initial dissociate begins to occur, carbon dioxide is injected into the formation. The carbon dioxide, i.e., the releasing agent, from carbon dioxide supply 34 can be introduced into casing 22 via a carbon dioxide pump 36. As the methane from the dissociated hydrate is produced, the injected carbon dioxide will form hydrate from the free water left as a result of dissociation. The carbon dioxide is injected in a controlled manner so as to selectively dissociate the methane-rich hydrates. As carbon dixoide is injected into the formation, it will both form a pure carbon dioxide hydrate from the free water and also exchange with the methane in the existing hydrate formation creating a more stable hydrate. The reformation of the hydrate from the carbon dioxide serves to sequester it and reduce the produced free water. Because the hydrate formation is exothermic, this also counters the cooling from the hydrate dissociation. In addition, because hydrate is formed, the loss in geomechanical strength of the hydrate-bearing sediments should be minimized.

The preferred embodiment of the present invention has been disclosed and illustrated. However, the invention is intended to be as broad as defined in the claims below. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described in the present invention. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims below and the description, abstract and drawings not to be used to limit the scope of the invention.

REFERENCES

All of the references cited herein are expressly incorporated by reference. The discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication data after the priority date of this application. Incorporated references are listed again here for convenience:

-   1. U.S. Pat. No. 7,222,673, Graue et al., “Production of Free Gas by     Gas Hydrate Conversion.” 

1. A method for the producing hydrocarbons from a subterranean formation containing gas hydrates, the method comprising: a. drilling a well into a subterranean formation; b. introducing a releasing agent into the well in a controlled manner to partially depressurize the well, wherein the releasing agent is more thermodynamically stable than the gas hydrates present in the formation; c. causing the releasing agent to contact the gas hydrates, thereby releasing the hydrocarbons in the well without melting the gas hydrate; d. selectively substituting the releasing agent for the hydrocarbons to thereby release the hydrocarbons from solid-state hydrate structure s without melting the gas hydrates, thereby providing substituted hydrates comprising the releasing agent bound with the solid-state hydrate structures that form hydrates, thereby releasing the hydrocarbons into the well without melting the gas hydrates; and e. reducing the pressure in the well to selectively dissociate the hydrates with a desired methane content.
 2. The method according to claim 1, wherein the releasing agent is selected from a group consisting of carbon dioxide, ethane, xenon, hydrogen sulfide, and mixtures thereof.
 3. The method according to claim 1, wherein the gas hydrate is a methane hydrate.
 4. A method for the producing hydrocarbons from a subterranean formation containing gas hydrates, the method comprising: a. drilling a well into a subterranean formation; b. introducing a releasing agent into the well in a controlled manner to partially depressurize the well, wherein the releasing agent is more thermodynamically stable than the gas hydrates present in the formation, wherein the releasing agent is selected from a group consisting of carbon dioxide, ethane, xenon, hydrogen sulfide, and mixtures thereof; c. causing the releasing agent to contact the gas hydrates, thereby releasing the hydrocarbons in the well without melting the gas hydrate; d. selectively substituting the releasing agent for the hydrocarbons to thereby release the hydrocarbons from solid-state hydrate structure s without melting the gas hydrates, thereby providing substituted hydrates comprising the releasing agent bound with the solid-state hydrate structures that form hydrates, thereby releasing the hydrocarbons into the well without melting the gas hydrates; and e. reducing the pressure in the well to selectively dissociate the hydrates with a desired methane content.
 5. The method according to claim 5, wherein the releasing agent is in liquid phase when contacted with the gas hydrates.
 6. The method according to claim 5, wherein the gas hydrate is a methane hydrate. 